The Current Trophic State and Water Quality of Lake Onego

Abstract—The trophic state and water quality of Lake Onegо have been assessed under climate warming and changes in anthropogenic load over the past 30 years. The water body retains its natural oligotrophic state during the summer stratification according to the concentration of Chl a. A small amount of readily mineralizable organic matter in water determines the low level of development of saprophytic bacteria, corresponding to xenosaprobic and β-oligosaprobic waters. Only Kondopogskaya Bay in Lake Onego is characterized by a higher trophic level (mesotrophic) and water saprobity (β-mesosaprobic) due to pollution by wastewater from the PPM and waste from trout farms. The local bloom of cyanobacteria in the open area of the lake was observed during the anomalous heating of the epilimnion in the summer of 2022 for the first time in the 50-year history of research. The increase in the concentration of humic substances in the water of the bays as a result of climate warming at this stage did not lead to a change in the level of saprophytic bacteria.